In recent years, the exhaust gas regulations relating to nitrogen oxides (hereafter abbreviated as NOx) and the like have become considerably more stringent, and satisfying these regulations has become a pressing issue.
Numerous methods have already been proposed for removing NOx within the exhaust gas from high-temperature combustion systems. Furthermore, in the case of the exhaust gas from large boilers and the like, which represent fixed emission sources for NOx, a selective catalytic reduction process that uses ammonia as a reducing agent (hereafter referred to as a NH3—SCR process) is generally employed in practical applications.
In this NH3—SCR process, NOx is reduced and converted to harmless nitrogen gas and water vapor in accordance with the reaction equations shown below.4NO+4NH3+O2→4N2+6H2O6NO2+8NH3→7N2+12H2ONO+NO2+2NH3→2N2+3H2O
The NOx removal catalyst used in this NH3—SCR process (hereafter also referred to as a SCR catalyst) typically employs a metal oxide-based catalyst such as alumina, zirconia or vanadia/titania, or a zeolite-based catalyst or the like. Furthermore, the use of a catalyst comprising titanium, tungsten, and at least one of vanadium, molybdenum and iron (patent reference 1), and the use of an iron ZSM-5 monolithic structure zeolite having a specific silica/alumina composition (patent reference 2) have also been proposed.
Furthermore, a pentasil zeolite (ZSM-5) with iron supported thereon has been proposed for removing nitrous oxide (N2O), which is a contributor to global warming, from exhaust gas (patent reference 3). Moreover, in a similar manner, a method in which contact with a β-zeolite with iron supported thereon at a high temperature of 350 to 500° C. is used for removing N2O from exhaust gas has also been proposed (patent reference 4).
However, none of these proposals relate to a method for purifying the exhaust gas from the diesel engine of a NOx mobile emission source such as a truck or bus.
In the aforementioned SCR process, if ammonia is used as the reducing agent, then because ammonia has a pungent odor, is toxic, and must be transported and stored using pressurized containers, problems of safety, handling and workability arise. Application of ammonia to a NOx reduction treatment system for the exhaust gas from the diesel engine of a truck or bus offers limited practicality.
Instead of using ammonia, compounds that are solid at ambient temperature but then decompose to generate ammonia, such as ammonium carbonate, urea, cyanuric acid and melamine can also be used. These compounds are preferred, as they offer favorable handling characteristics and suffer no safety problems such as ammonia leakage, and of these compounds, urea is particularly practical as it is also inexpensive.
Urea generates ammonia via a thermal decomposition reaction (i) and a hydrolysis reaction (ii) represented by the reaction equations shown below,(NH2)2CO+H2O→NH3+NHCO  (i)NHCO+H2O→NH3+CO2  (ii)and the generated ammonia then acts as a reducing agent in the manner described above, thereby effecting a reduction reaction of the NOx within the exhaust gas. Because urea is a solid, suitable methods of supplying the urea to the above reaction system include methods in which the urea is sprayed into the gas as an aqueous solution.
[Patent Reference 1]
Japanese Laid-open publication (kokai) No. Sho 50-128681
[Patent Reference 2]
Japanese Laid-open publication (kokai) No. Hei 9-103653
[Patent Reference 3]
Japanese Laid-open publication (kokai) No. Hei 7-60126
[Patent Reference 4]
Japanese Laid-open publication (kokai) No. Hei 8-57262